CVD processes for manufacture of synthetic diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, diamond can be deposited.
Atomic hydrogen is believed to be essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for diamond film growth via a chemical vapour deposition (CVD) process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
A useful overview article by Silva et al. (Paris University's LIMHP group) summarizing various possible reactor designs is given in the previous mentioned Journal of Physics (see “Microwave engineering of plasma-assisted CVD reactors for diamond deposition” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364202). Having regard to the patent literature, U.S. Pat. No. 6,645,343, EPO480581 and US2010/0189924 disclose various reactor designs including systems in which process gas is injected into the plasma chamber at high velocity to establish convective transfer of activated gas species from the plasma to the substrate in order to increase growth rate of a CVD diamond film and/or improve thickness uniformity of the CVD diamond film.
Doping of synthetic diamond material during CVD synthesis is also known in the art. Common dopants in diamond material which may have some desirable use include boron, nitrogen, silicon, sulphur, phosphorous, lithium and beryllium. Synthetic boron doped diamond material is of particular interest as boron doping can, for example, make the synthetic diamond material semi-conductive or, at high doping levels, full metallic conduction can be achieved. Synthetic boron doped diamond material finds applications that range from mechanical applications to electronics and sensors.
There is a need to grow synthetic diamond material which contains a uniform concentration of dopant to maintain consistency of product. For example, in boron doped polycrystalline diamond it is desirable to grow large area (e.g. greater than 120 mm diameter), thick (e.g. great than 0.5 mm), free-standing polycrystalline diamond wafers which can be processed using electric discharge machining (EDM) methods. In order to achieve this, the boron concentration needs to be high enough to ensure a reasonable and viable cutting rate, but not so high that it begins to degrade the material properties. Furthermore, the boron concentration must be within these limits over the majority volume of the disk.
A similar argument applies to single crystals, for example wherein a plurality of single crystals might be homoepitaxially grown in a single growth run. Specifications on the boron set by applications that include electronics require all of these single crystal diamonds to contain similar boron concentrations.
There is also a need in some methods (particularly in single crystal {100} oriented growth) to find routes to achieve higher boron concentrations necessary for example, for metallic conduction.
A significant amount of work has been performed in this field in relation to boron doped polycrystalline and single crystal diamond material. For example, EP 0 822 269 B1 discloses the basic CVD chemistry required for achieving boron doping. EP1463849 teaches how to achieve uniform boron doping over a single crystal of synthetic CVD diamond material by utilizing a diamond substrate having a surface substantially free of crystal defects.
J. Achard, F. Silva et al. also discuss boron doping of CVD diamond material using a reactor as described in the previous discussed Silva et al. paper (see “Thick boron doped diamond single crystals for high power electronics”, Diamond & Related Materials (2010), doi: 10.1016/j.diamond.2010.11.014). Here, the effect of boron concentration in the reaction gases and microwave power density is discussed in relation to boron doping of single crystal CVD diamond material. It is described that in order to increase the level of boron incorporation into a single crystal CVD diamond film it is necessary to increase the amount of diborane added to the reaction gases but for [B]/[C]gas ratios above 5000 ppm the plasma is unstable due to formation of soot that accumulates and prevents deposition longer than a couple of hours, and thus prevents the growth of thick films. It is also described that high microwave power densities are desirable for rapid growth of CVD diamond films but that higher microwave power densities result in lower boron incorporation. As such, it is concluded that a compromise must be reached by using a mid-range microwave power density (specifically disclosed as 60 Wcm−3) and a [B]/[C]gas ratio of 5000 ppm to grow a 300 μm-thick heavily boron-doped film (1020 cm−3) from which a freestanding plate can be formed.
It is an aim of certain embodiments of the present invention to provide a method and apparatus which is capable of achieving more uniform doping of CVD diamond material over large areas of, for example, polycrystalline diamond material and/or over a large number of single crystal diamonds grown in a single growth run. It is also an aim of certain embodiments to achieve higher levels of doping such as high boron doping concentrations for electronic and sensor applications. It is a further aim to achieve uniform and/or higher levels of doping while simultaneous achieving good growth rates given that some dopants such as boron have a tendency to reduce growth rates.